1. Field of the Invention:
This invention relates to a gas sensor capable of stably measuring gas concentration or gas partial pressure in a solution, living body or gas by the potentiometric response of an electrode. More particularly, the invention relates to a gas sensor for measuring carbon dioxide, ammonia gas, etc.
2. Description of the Prior Art:
A glass electrode is well-known as a gas sensor for sensing such gases as carbon dioxide gas and ammonia gas. The glass electrode has a thin internal solution layer retained between the sensitive portion of a pH sensor and a gas-permeable layer. Gas molecules which pass through the gas-permeable layer dissolve in the internal solution and change the pH thereof, which change is sensed by the pH sensor. However, since the electrode used in a sensor of this type is a glass electrode, the sensor breaks easily and is difficult to miniaturize. To eliminate these drawbacks, the inventors have disclosed a number of solid-state pH sensors, as described in the specifications of Japanese Patent Publication Nos. 59-281076, 60-35691, 60-53308 and 60-93176.
However, it has been found that even a gas sensor employing the aforementioned solid-state pH sensor experiences a deterioration in the ionic characteristics (particularly the ion sensitivity) of its electrode about one month. Another disadvantage is that the gas sensor is not suited to continuous monitoring because of a slow response with regard to the change in solution temperature.